Mass spectrometer



June 3, 1952 H. w. WASHBURN 2,598,734

mass SPECTROMETER Filed Dec. 11, 1945 2 SI-IEETS-SI-IEET 1.

7'0 VACUUM PUMPS INLET TUBE II AMPLIFYING AND RECORDING APFHRA TU:

"LE T ELECTRODE IZ ELECTRON BEAM 1.!

TARGET Z/ SHIELD Z7 suppnssson mm 4/ uusanreo ION saw 5 swam ANALYZER TUBE II 10 mcquu ms SAMHE TUBE II AMPL IF Y/NG AND RECORDING APINRA TU! INLET DEAN IJ TARGET ll SHIELD Z7 -SUPPRE8$0R GRID ENVELOPE ZJ 4mm? rue; Ia

IN V EN TOR. HAROLD K. WASHBUR/V ATTORNEYS I June 3, 1952 H. w. WASHBURN 2,598,734

' MASS SPECTROMETER Filed Dec. 11, 1945 I 2 SHEETS-SHEET 2 T0 VACUUM PUMPS INLET TUBE II AMPL IF Y/NG AND flECORD/NG APIHRATUS I2 BEAM I3 TARGET 21 I6 SHIELD 27 ENVELOPE ZJ ANALYZER TUBE II IN VEN TOR.

ATTORNEYS thehi eso .ot oet rol' is Patented June 3, 1952 W 5: FF RQMFF 'ER.

,\ both oehe l ossignor to Engineering Corporation; Pasa iioration'ofcolitqlnio,'

Applieotion nee-m t 11, 19.45, sar No. 634,363

This invention is concerned with moss spectrometers. and proyides improvements whi h per}; mit the. sup ressionof sow-called f-fgh a mass spectrogram and also, the: mimm z he of zero shift." Moreover. theinyeo ohmakes it possible to determine the mass spectrum oil only thoseions havin k netic ener y n ex e 9 a given m n n hus. permits oe e omin t o ofthe energy distribution'o onsn duc d, role a sample. Although applicable to mass $1 .9? trometers in enera-1, it is ertiou o lx. hostel, to those oi the; type of whioh ettin o qhe the analyzeris, conducted in or su stantia l 99. t t ma netic fie d, iocuosioe or ions the c011 ton e e trode bein acoomnli od li V .1 th po entia which, prope s the. 1 2. tottorg the collector. Y

A mass spectrometer is an apparatus for; ion; izing molecules and sorting them e, magnetic field accordin t specific. mo e- Iheeherot comprises an. ionizat on ohem e in W o hetero eneous mixture. of molecu es 71. v 59. usually. by lectron homhot ment .1." itmehm efieotian analyzer in which, a ma n tio 12 established, a pl rality of electrodes; im pressing a potential out the ions he chem e; and propel in them as a hetero eneous he the. analyzer, (wherein the ions, re s parated into a plurality of diyergent homo eneous home) a target or collector electrode at the exit 9? 31 analyzer, means for iocussing any. one; 9;. the homogeneous beam on the target t the?- Eh; varyin the propellin potential o the mos etio held and moons o retqtd hs hs fomhl the ourre co lected a the tor e 9t! c 35 oorneopohd o o the. h1 hQIhQtthQPt heoms o e hoioee'o tht ohhhqohq oteesh 2"? theseveral kinds of ions formed from the samp e. The re ord o he se eral 9H 9 oltotrmo o opootro 5% meet P hol et Yo oh oh ht. t o e99 o't te mes ee t hat reootdihe e lvehot et temhho o t. to e; Pesto i i o her 139 h. Q; 45 he li e a Po ts orreop hdihe teen-Ethel $19 I n8 he ted rom the samp e height otfoe h to! ohoh q eel a oenhtetiye mee oro otthe omohht correspondi ind of ionies ever in mom? h: st nces; he. soeot os om i oo ho otesl by the prese c of hos ofl ismel peosshroh eh'l due to the D1Q o m toetehle io s or to. ions that have been slowed throu h tqllioion with occluded gasmolecules on interior metalli h tool-he 2 urto es oi the pect omet r d coheeq eh l orm, oroo ed e oh o y ys Se Meet Spectrav and. I ot pes b Aston ec nd dition len then Gr e C p- .'-63- "$92 2 t me t e. shost 'op eot S para as m l t}? ivel wide eeks. o bum s i i f m the bas ine At ther-t mes the hos de ith one me or nc ea e rue- .eohs hat represent ident fiab stable omponents At St l othe times the ghost takes. the form of a, very wide sha low. pea 0 fumn ha ove es e e r i h oks thus ivin t mpr n ha e he? o datum ne h s en an os i tom litoh ins the measurement o r e Pe k, heigh sh sho t. he ghosts m opo o ooh r ohr or. oterlonpih true peaks, "theyinteriere with accurate mter e t h f m s sp a by vin he ta e? im es on hat he te a n 5 small Pro ortion o o om onent wh actually is ehseht or. in the ose of ver ap, e ex ve v ue or th quanti o o omp e that" t res nto s. o esul o myih eo ieo p s e espective of theories as to 'wha t causes ghost sfi I haye developed improyements"in'massTspectnomet zx wh ch e m ht'ehmi or ghosts ess s ect um a d he on o bas 1 2%? hift w o di e abovfis lit d to be merely another form of ghost.

, 4s 11 have olr eedy, indicated, the ions produced n the ioniza ion c a e are Pi peue 'e trical' potentials established between a phi rolit}; of" electrodes disposed in the path of the ion beg n. if; spectrometer oe-yv be constructed with only tvgo such electrodes; but in orderf to form borrow beams which are easily focussed on the target, more than two propelling electrodes hould be em lo d Fo exo o o the r-ome er hov o Pusher e e t e i he 99,- ization h mber it lf, an interm diate e ectrode ha n a firs i ($11 at t e t of e ionizer tion chamber and a terminal electrode at the entrance of the a alyzer ha n a econ s (S2,) in line wi h S1 an h pu he electrode- A potential established between the pusher and the int rme te e c d forces the i n to flow. out of the nization chamber. throu h-the aperture 5;, Add ti nal p t n a imp ies he: tween t e intermed a e and ermina eleotrooeo urt er acce er t s. the i s o e hropelo hem throu h So into he anel ze he wo lit oid 1o ool imetih the boom o t n hto thooho xher. flfhe otentials impresse betwe n the pus er and or and etween oh $2 todi eeht. he

. t eooe to, he analyzer should he iii he s me iii:

rection, so as to aid each other in propelling the;

tan

ions, and customarily thepusher is positive with a 'tential, and there are advantagesto both such arrangements. My invention is applicable to either.

I have discovered that by providing an auxiliary potential producing means for imposing in'" the neighborhood of the target an auxiliary potential opposing the movement of ions from the analyzer to the target, it is possible to prevent the appearance of ghosts. Thus, by providing an" interconnection between the circuit which supplies the propelling potential for. the ions and the l target or a suppressor grid disposed in the front of the target in the path of the ions issuing from the analyzer with auxiliary potential producing means in the interconnection; ghosts-may be'elimmated. There should be means for suitably adjusting-the potential at the target (Fig. 3) or grid '(Figs. 1 and 2). By suitably adjusting this potentiaLitis possible to prevent ions that have no kinetic energ'yat the point of their formation,

say in the path of the electron beam inthe ionization chamber, from reaching th target. In such case, a spectrum is produced from only those ions that have initialkinetic energy. Such a result. is useful in analyzing the conditions under which ionization 'occurs. Moreover, by adjusting the potentialat'the target orgrid to other values, it is possible to suppress ions having less than any given kineticenergy so that the spectrum produced represents only those ions having kinetic energy above a certain level. This is desirable,

not I only for the determination of -energydistributionfof an ion beam but also because'in many instancesit permits a spectrufn' to be simplified by the elimination of certain peaks whichmaybe without significance in the particular case.'-- This. .1 permits r'nore rapid calculation of numericalre-v sults from thespectrogram. Y I Especially -whenthespectrometer is of the'type in which the magnetic field is kept constant-andthe'shifting of beamswith respect to the target accomplished by variation of the propelling po tential, it is; desirable to connect a condenser, preferably a variable one, between the target and 'thelcircuit which producedthe propelling potential. In this fashion, the capacitycoupling between the'target and suppressor grid in one case a (or between the target and the end of the analyzer'tube if no suppressor grid is interposed) may be neutralized. This prevents erratic creep of the baseline of the spectrum. Ina mass spectrometer in which an intermediate propelling electrode (saythe electrode 0011- taining S1) is grounded, the condenser may be,

connected directly between this electrode and the target.

If the terminal propelling electrode (say that containing S2 and connected to the analyzer) 'is grounded'the condenser should be connected into the circuit supplying the propelling potential with aresista'nce in the circuitbetween the condenser 'andthe'ground. In such an arrangement, the

4 battery, means'being provided for varying the potential at the suppressor grid, as described hereinbefore.

In either arrangement, i. e. with S1 or S2 grounded, the target preferably is protected by a shield or grid. In most types of apparatus, the h d. h u .d be s gdi ound, .F ample, in a. mass spectr0m'etei' -in which the target is connectedto groundthrough'a'high resistance which is shunted by an amplifier that is a "part of the indicating or recording apparatus, the shield should be connected directly to ground,

, by-passing the large resistance.

rangementthe' condenser which connects the target to the circuit supplying propelling potential "shouldbe connected ahead of the large resistance shunted by. theamplifier.

In such an ar- These and other aspects of my invention are "described in detail hereafter with reference to the accompanying drawings in which: Fig. 1 i's-a schematic diagram illustrating amass spectrometer equipped for the practice of the invention with a terminal propelling electrode, i. e. that at the analyzer, connected to ground:

Fig. 2 is a diagram illustrating a modification of the arrangement of Fig. 1; and r Fig. -3 is a further diagram illustrating the application of the invention to a spectrometer, having an intermediate propelling electrode rounded. I J ,7

Referring to Figal, the spectrometer per se comprises a cylindrical head or block ID, the rear of which connects with a sample inlet tube ll made of'insulating material. "Within the head, in the'path of thegas from-theinlet tube, there is a pusher or inlet electrode [2 in the form of a conductive plate insulated from the rest of the apparatus. a g A Anelectron beam I3 is produced within the headin-the space immediately below the pusher by meansof an electron gun (not shown). The spacein the head "in which the electron beam passes is'known as the ionization chamber 14. The front of the ionization chamber is formed by a'n intermediate-- propelling or collimating is bent in asemi-circle and encloses an analyzer chamber l9. i I V In the operation of the spectrometer, a Sample Of as to be analyzed is'admitted to the ionization chamber through the gas inlet tube. Molecules of' the gas sample are'b'ombarded by electrons; of the beam and are thus ionized. The resulting ions are propelled as an unsorted ion beam into the analyzer tube by means of electrical potentials established between the pusher and the two collimatingelectrodes. Thus, the slits S1 and S2 through which the ions pass cause the ions to emerge'into the analyzer asa thin ribbon,'a cross-section of which is defined approximately by the cross-section of the'slits. In the'analyzer the unsorted ion beam is separated into a plurality of diverging homogeneous ion beams B1, 32,133 by means of a magnetic field produced in'the analyzer tube'as well as in the amaze:

head of the apparatus by an electro magnet (not shown). I

Any one ot'the diverging ion beamszzmay. be focused'on an exit slittlfl. at the fariendiofithe analyzertubeiso thatit passes-therethro'ugh and strikes anion collector or targetflzt.

The head of the spectrometer and the attached analyzer tube is are enclosed within. an envelope 23 through the wallet whichthe sample inlet: tube projects: 'A;.h'iglr.degree oi vacuum is .maintainedjwithinj the envelope by means. of vacuum pumps. (not shown) connected to ages outlet 24-. 1' i he tar et or co lector tr d is c nnected to ground' through ahigh resistance 15. This e st n isshun y ani mp i nsj d ndieat -0rc rd ne ar u -1e Pre ra this amplifier is a current amplifier-oi thenega tive' feed-backtype so as to minimize the effect of the high resistance. V r

vWithin the envelope and substantially surrounding the target. is a metallic shield 21 in the formof a cylinder with. a disk at its end interposed between the end of the analyzer tube. and the target. .This disk has. an aperture or slit 28 in line with'the target andthe exit slit. of the analyzer tube. j

v The electrical circuit which produces the propelling potentials within the head of the mass spectrometer is shown atthe' left of. Fig. 1. A battery or other direct current supply 30 is connected in an energizing network to the ends of a potentiometer 3|. The positive end of this potentiometer and its slider. are connected through. a switch 32. in this network to a. condenser 33. The negative end of the potentiometer 3| is connected to the opposite side of condenser 33. A potential dividing network is connected across this condenser and takes the form of apotentiometer 34. connected in series with a second resistance 35.. A lead connected intermediate the resistances 34 and 35 is connected to the terminal propelling electrode [6 and to ground. The potentiometer 34 has a slider 34A which is connectedto the intermediatepropell-ing electrode l5, which. contains the slit S1. Since the terminal. propelling electrode I6 is electrically connected to the analyzer tube, this latter is also a grounded. I

The pusher electrode is connected to the potential dividing network between the condenser and potentiometer 34. This represents conven-' tional means for connecting a power supply to the electrodes of a mass spectrometer and thus forms no part of the present invention. Substantially similar means are described and illustrated in United States Letters Patent, 2,413,668. issued to me on December 31, 1946. v

The path of the ion beams B1, B2. B3 with respect to the exit slit and the target maybe changed either by varying the magnetic field or the propelling potentials. Thev latter method is preferable and may be practiced as follows: The'switch 32 in the energizing network is closed andthe condenser 33 is charged to an.

appropriate level. Then the switch is opened and the condenser is permitted to discharge through the potential dividing network and through the propelling electrodes in the head of the spectrometer. In this way, the propelling potentials between the several electrodes.v grad.- ually decay while maintaining relative'values. The potentials in the head may be adjusted relative to each other by suitably adjusting the slider of the potentiometer 34., such adjustments 6, being made to (give optimum results at thetarget. A; variable-condenser 40: is connected fromthe target: to the circuit supplying the propelling: no: tentials. The. condenser is. connectedz'betweenzthe target. andthelarge resistance 25; in such fash ion that thepath to ground is; via. the target, the condenser and, the resistance 35,.

A suppressor grid 41 is interposedrbctween; the end of the analyzer andtheshield surroundin the target. It is 'apertured topermitthe passage of ion beams, this. aperture being in line with the exit, slit and the slit 281 in the. shield. 'ThflISLlp-r pressor. grid is connected to the intermediate electrode 15, this connection being madethrough an auxiliary potential source which, in the ine stantcase, comprises a potentiometer 5D shunted by a 'batterytl'. By. varying the setting of the slider of the potentiometer more or less. of the auxiliary potential may be impressed on the sup: pressor grid with the advantages already; described. Thus, the suppressor grid may be maintained at the potential of the intermediate electrode or adjusted to suit operating conditionsfor the suppression ofions having: more or less kinetic-energy. a The apparatus of 'Fig. 2 is the same as that of Fi'gil like parts being indicatedby like numhers, save in the followin particularszThesuppressor grid, instead. of being connected; to-thc intermediate electrode, is connected into the note work which supplies the propelling potential; Specifically, the suppressor grid is connected through the slider of the potentiometer 5.0. (the potentiometer being shunted by a battery 5.!) through a tap 43 which is connected to the upper portion of the potentiometer 34 below a smail portion 34B of the-resistance of this potentiometer. The position ofthe tap 43. is chosen to takcofi to the suppressor grid a potential. op.- proximating thatexisting in the ionization chamber in the region of the electron beam. This latter potential depends upon the geometry of the ionization chamber. If the path of the electron beam is halfway between the pusher and themterrhediate electrode, the potential at thatpoint will be the average of the respective electrodes, so that the resistance 34B should be aboutv equal to the resistance 340 between the tap 43 and the slider 34A. With this arrangement, even though no auxiliary potential be'introduced from the battery 5!, ions that lack initial kinetic energy will be suppressed.

To summarize, the result of the circuits illustrated in Figs. -1 and 2, the battery- Sl or other D. C. potential sou-roe may be employed to pro 'duce at the suppressor grid 2. current of'such strengtndirection and potential that it just re pels the ions responsible for ghosts and prevents them from passing through the grid toward'the target. The same circuit arrangements permit adjustment of the potential at the suppressor grid to a value such that only those ions with initial kinetic energy will pass to the target. Other adjustments may be made so that only those ions having certain amounts of kinetic energy will pas the grid and strike the target. a The apparatus of Fig. 3 varies in several par.- ticulars from those of Figshl and. 2, but. gen.- eral is adapted to accomplish the same purpose. Parts that are common to the three types of. anparatus are indicated by like numbers. However, it should be noted that the condenser 40 is di. rectly connected between the target and the inlet or pusher electrode, instead of being connected to ground through the resistance .35. Secondly,

r 1 potential.

, pression'required.

electromagnet (not shown) is provided.

Q potential;

" 'The principles the intermediate electrode. I containing the .slit 1 S1 is groundedand the terminal propellingelec- 1 trode l5 containing the slit S2 is made negative 5 with respect to the intermediate electrode;:'The analyzer tube is electrically connected to the terminal propelling electrode. and is at the same Thirdly, the suppressor grid-is elim- 'inated and an auxiliary potential to oppose move- I ment oftcertain'ions .from the analyzer to the 5 target is imposed on thetarget itself, by means Q of a variable auxiliary D. C. potential source connected between the target and ground. Thus, the slider of a potentiometer '52 is connected to the targetand'this potentiometer is shunted by a battery 53, the lower end of the potentiometer (as shown in Fig. 3) being connected to ground.

By appropriately setting this potentiometer 52,.

the potential at the target-may be setvto a value appropriate for the particular type of ion sup- In all three .types of apparatus illustrated, an

analyzer tube and the ionization chamber are both disposed between the poles of the magnet so that 1 the lines of force between the polesare substantially perpendicular to the plane of the drawings.

to the exit;slit of the analyzer, may be accomplished by varying the magnetic field or, as described hereinbefore, by varying the propelling Variation of the propelling potential is, however, preferred since variation of the magnetic field when it encloses, the ionization cham- 1 her. may bring about erratic results.

'In the operation of the various apparatus i1- lustrated, the capacitance of the condenser betweentarget and propelling circuit is adjusted to neutralize the capacity, coupling between the The r 8 target. by an opposing potential maintained in the neighborhood of the target. This opposing potentialis obtained by means of any one of the three combinations illustrated, in'that a potential of required value opposing that propelling the ions is produced at the target or at the suppressor grid." v j Iclaim: .f .v Q. .1 1. In a mass spectrometer having an ionization chamber, meansfor ionizing molecules in the chamber, an analyzer, a circuit for producing a potential to propel ions-formed from the'molecules as a beam into the analyzer, means for sortingthe ions of the beamin the analyzer according to'specific mass anda target forcatching the sorted ions, the combination which comprises means connectedto' the circuit 'for producing in the neighborhood of the target a potential opposing those of the sorted ions, and means for vary-" ing the opposing potential.

2. In amass spectrometer having an ionization chamber. means for ionizing molecules in the chamber, an'analyzer, a circuit for producing a target and suppressor grid (Figs. 1 and 2) or between the target and the end of the analyzer tube (Fig. 3). In general, this'adjustment will serve thereafter for the analysis of subsequent unknown samples and readjustment will seldom be necessary. r V

underlying the suppression of ghosts with the several types of apparatus of the e invention are as follows:

. 1A metastable ion may be formed by electron v bombardment, for example when a molecule of normal butane of mass 58 is subjected to bombardment, it may lose an electron and sometimes :becomes metastable, so thatit maylater 'lose a When this methyl group-and attainmass 43. methyl group is lost the vicinity of the second slit, which is experimentally, shown to be the case, kinetic energy of the ion is reduced and the resulting ion behaves asthough it had mass 32, in the sensethat it takes the same path in the analyzer as .do oxygen ions. Such ions, however,

"Slow ions may result frcm inelastic inter- I action with the fields of the instrument, includ- :ing' fields of very small dimension in the vicinity I of the walls'of the instrument, or of other ions,

withithe result thatthey-travel with low energy tected'and-may be preventedfrom reaching the of secondary beams arecharactermove with less energy than do the oxygen ions.

potential to propel ions formedfrom the molecules as a beamiinto the analyzer, means for sorting'the ions of the beam'in the analyzer according to specific mass, and a target for catching the sorted ions, the combination which comprises shielding means disposed around the target, auxiliary potential-producing means connected to the circuit for producing in the neighborhood of the targets; potential opposing the movement of the sorted ions toward the target and including an auxiliary electrode "disposed adjacentthe target exteriorly'of said shielding means. v 1 c H 3; In amass spectrometer having an ionization chamberpmeans for ionizing molecules in the chamber, an analyzer, a circuit for producing a potential to propelions formed from the molecules as a beam into the analyzer, means for sorting" the ions of the beam in the'analyzer according to specific mass,-and a target for catchingthe sorte'dions, the combination which comprises shielding means disposed'around the target and connected to ground, a variable condenser through which the target is connected to the circuit, and auxiliary potential-producing means connected to the circuit for producing in the neighborhood of the target 'a potential opposing the movement of the sorted ions toward the target. V

4. In a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamben'an analyzer, a circuit for producing a potential to propel ions formed from the molecules as a beam into the analyzer, means for sorting the ions of the beam in the analyzer according to specific mass, and a target for catching'. the sorted ions, the combination which comprises shielding means disposed around the target, a condenser connected between the target and the" circuit, auxiliary potential-producing means connected to the circuitfor producing in e the neighborhood of thetarget an auxiliary potential opposing the movement of the sorted ions toward the target, and means for varying. the auxiliary potential. h

- 5. In amass spectrometer having an ionization chamber, means for'ionizing molecules. in the chamber, an. analyzer, a .circuit for producing a potential to propel ions formed from the moleculesas a beam intothe analyzer, means forsorting the ions of the beam in the analyzer accord-'- mg to specific mass, and a target for catching aways-4 potential to propel ions formed from the mole- 7 cules 'asa beam into the analyzer means for. rting thefion's of the beam in theafi'alyzer' ac'cortling to specific mass, and a target for'catching the sorted ions, the combination which comprises shielding means disposed around the target and connected to ground, a resistance through which the target is connected to ground, indicating means shunt-connected across the resistance, a condenser connected to the target in parallel with the resistance and also connected to the circuit, and auxiliary potential producing means connected to the circuit for producing in the neighborhood of the target an auxiliary potential opposing the movement of sorted ions toward the target.

'7. In a mass spectrometer having a chamber in which molecules are converted into ions and an analyzer communicating with the chamber in which the ions are sorted according to specific mass, the combination which comprises a first propelling electrode disposed in the chamber, an intermediate propelling electrode and a terminal propelling electrode both disposed along the line of communication between the chamber and the analyzer, the terminal electrode being connected ions, a condenser connected between the target and the circuit and to ground through a resistance in the circuit, an auxiliary potential source, and a suppressor grid disposed between the analyzer and the target and connected to one of the propelling electrodes other than the terminal electrode through the auxiliary potential source.

8-. Apparatus according to claim 7 provided with a resistance through which the suppressor grid is connected to the first electrode.

9. In a mass spectrometer having a chamber in which molecules are converted into ions and an analyzer communicating with the chamber in which the ions are sorted according to specific mass, the combination which comprises a first propelling electrode disposed in the chamber, an intermediate propelling electrode and a terminal propelling electrode both disposed along the line of communication between the chamber and the analyzer, the terminal electrode being connected to ground, a circuit connected to thethree propelling electrodes for establishing a potential between the first and intermediate electrodes with the first electrode positive with respect to the intermediate and a potential between the intermediate electrode and the terminal electrode with the'l'atte'r' negativewith respect to the intermediate electrode, a target electrode operatively associated with the analyzer roi- "collecting-the -sorted ions, and a "condenser connected between the target electrode and the circuit arid-to ground "through a resistance in the cire'uit, an auxiliary potential'source, and a suppressor grid interposed between the analyzer andthe tai'fget ewe tide and eo heetee to a portion-of the circuit that positive respect'to the terminal electrode through theauxiuary potentim seurce. 4

10. Apparatus according to elaiine in the condenser is variable. i

11. In a mass spectrometer provided with means for formingiona-anannyier, aciifcuit 'for producing apotentim to propel-the ions t u g'li thean'alyzer; and 'a' target eltrbde 'f 1f reeeiviii'g' the "ions thus-propelled, the combination 'wh'ieh comprises an auxiliary potential producing means for imposing in the neighborhood of the target an auxiliary potential opposing the movement of ions from the analyzer toward the target and including an auxiliary electrode disposed adjacent the target, and means for varying the auxiliary potential.

12. In a mass spectrometer provided with means for forming ions, an analyzer, a circuit for producing a potential to propel the ions through the analyzer, and a target electrode for receiving the ions thus propelled, the combination which comprises an auxiliary potential producing means for imposing in the neighborhood of the target an auxiliary potential opposing the movement of ions from the analyzer toward the target, means for varying the auxiliary potential, and a condenser connected between the target and the circuit.

13. In a mass spectrometer provided with means for forming ions, an analyzer, a circuit for producing a potential to propel the ions through the analyzer, and a target electrode for receiving the ions thus propelled, the combination which comprises an auxiliary potential producing means a for imposing in the neighborhood of the target an auxiliary potential opposing the movement of ions from the analyzer toward the target, means for varying the auxiliary potential, and a variable condenser connected between the target and the circuit.

14. In a mass spectrometer provided with means for forming ions, an analyzer, a circuit for producing a potential to propel the ions through the analyzer, and a target electrode for receiving the ions thus propelled, the combination which comprises an auxiliary potential producing means for imposing in the neighborhood of the target an auxiliary potential opposing the movement of ions from the analyzer toward the target and including an auxiliary electrode disposed adjacent the target, means for varying the auxiliary potential, and a shield disposed between the target and auxiliary electrode.

15. In a mass spectrometer provided with means for forming ions, an analyzer, a circuit for producing a potential to propel the ions through the analyzer, and a target electrode for receiving the ions thus propelled, the combination which comprises an auxiliary potential producing means for imposing in the neighborhood of the target an auxiliary potential opposing the movement of ions from the analyzer toward the target and including an auxiliary electrode disposed adjacent the target, means for varying the auxiliary potential, and a shield disposed between the target I and the au ar l ct o an jc n ie {mass spectrometer provided 1 with I nieans iforjor-ming ions; an analyzer, a circuit for producinga potential to propel the ions through the analyzer, and a target electrode forreceiving thensrthus propelled; t o on which 1 comprises-a'suppressorgrid disposed between the analyzenand the target,=.an auxiliary potential for producing a potential to propel the ions I through the analyzer, and a target electrode for awem 12 receiving the ions thus propelledthe combination which comprises an auxiliary potential producin means connected ito the target ior'imposing in the neighborhood of the target an auxiliary potential'opposing the movement of ions from the analyzer toward the target, and means for varying the auxiiiarypotential. HAROLD.

' REFERENCESCITEI') j" p The following references are of record in the file oithis patent; a

W. WASHBURNJ- UNITED-STATES'PATE Number Name r j Date" v 2,370,673 Langmuir Ma r.,6, 1945 .2,4 12,359' V o er ;IDec.flI0,j1946 

